The present invention relates to selective directional couplers of the type composed of two preferably parallel waveguides which are to be coupled together.
Directional couplers whose coupling degree depends on the frequency or wavelength of the electromagnetic oscillations to be coupled out are required, for example, for carrier frequency data transmission of several channels in frequency multiplex in one and the same transmission medium. A typical application is two-way voice operation with optical signals on a glass fiber employing wavelength multiplexing. In this case, as well as in other applications with lower carrier frequencies transmission in one direction takes place at a different light wavelength than in the opposite direction. At the end points of such a fiber path, or at the repeaters, transmission takes place, as shown in FIG. 1 hereof, through a selective directional coupler at a wavelength .lambda..sub.0 and reception occurs from the opposite direction at another wavelength .lambda..sub.e.
The selectivity of the directional coupler is such that the entire transmitting power at .lambda..sub.0 is fed into the fiber connection and the entire incoming power at .lambda..sub.e reaches the receiver. The selectivity of the directional coupler, supported by its directional effect, moreover reduces near-end crosstalk, so that even with high transmitting power only a disappearingly small portion of the transmitted power reaches the associated receiver.
As a further advantage of such a transmitting-receiving duplexer, the fundamental mode received from the fiber connection, which for this application is a single mode fiber, can have any desired polarization since it passes through the selective directional coupler without coupling.